Pixel with differential readout

ABSTRACT

An imager in which two adjacent pixels share row and reset lines and a row selection circuitry while the output transistors of the two pixels are configured as a differential amplifier. In operation, both pixels are reset at the same time, causing differential reset signals to be output from the amplifier. The charge from the first pixel is readout and a differential pixel signal for the first pixel is output from the amplifier. Because the reset and pixel signals are differential signals generated within the pixels, they are free from common-mode noise. Correlated double sampling can be used to obtain the pixel output value, which is also free from common-mode noise, from the differential reset and pixel signals. The second pixel may be readout in the same manner. Because the two pixels are sharing circuitry, the pixels have decreased fill factor and complexity as well.

This application is a divisional of application Ser. No. 11/284,818,filed Nov. 23, 2005, which is a divisional of application Ser. No.10/684,471, filed on Oct. 15, 2003, which are hereby incorporated byreference in their entireties.

FIELD OF THE INVENTION

The invention relates generally to imaging devices, and moreparticularly to an imager pixel with differential readout.

BACKGROUND

Imaging devices such as complementary metal oxide semiconductor (CMOS)imagers are commonly used in photo-imaging applications. A typical CMOSimager circuit includes a focal plane array of pixel cells. Each one ofthe cells includes a photoconversion device or photosensor such as, forexample, a photogate, photoconductor, or photodiode, for generating andaccumulating photo-generated charge in a portion of the substrate of thearray. A readout circuit is connected to each pixel cell and includes atleast an output transistor, which receives photo-generated charges froma doped diffusion region and produces an output signal that is read-outthrough a pixel access transistor.

One typical CMOS imager pixel circuit, the three-transistor (3T) pixel,contains a photosensor for supplying photo-generated charge to adiffusion region; a reset transistor for resetting the diffusion region;a source follower transistor having a gate connected to the diffusionregion, for producing an output signal; and a row select transistor forselectively connecting the source follower transistor to a column lineof a pixel array. Another typical CMOS imager pixel employs afour-transistor (4T) configuration, which is similar to the 3Tconfiguration, but utilizes a transfer transistor to gate charges fromthe photosensor to the diffusion region and the source followertransistor for output.

Exemplary CMOS imaging circuits, processing steps thereof, and detaileddescriptions of the functions of various CMOS elements of an imagingcircuit are described, for example, in U.S. Pat. No. 6,140,630, U.S.Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652,U.S. Pat. No. 6,204,524, and U.S. Pat. No. 6,333,205, all of which areassigned to Micron Technology, Inc. The disclosures of each of theforgoing are hereby incorporated by reference herein in their entirety.

FIG. 1 illustrates a typical four transistor pixel 50 utilized in CMOSimagers. The pixel 50 includes a photosensor 52 (e.g., photodiode,photogate, etc.), floating diffusion node N, transfer transistor 54,reset transistor 56, source follower transistor 58 and row selecttransistor 60. The photosensor 52 is connected to the floating diffusionnode N by the transfer transistor 54 when the transfer transistor 54 isactivated by a control signal TX. The reset transistor 56 is connectedbetween the floating diffusion node N and an array pixel supply voltage.A reset control signal RESET is used to activate the reset transistor56, which resets the photosensor 52 and floating diffusion node N as isknown in the art.

The source follower transistor 58 has its gate connected to the floatingdiffusion node N and is connected between the array pixel supply voltageand the row select transistor 60. The source follower transistor 58converts the stored charge at the floating diffusion node N into anelectrical output voltage signal. The row select transistor 60 iscontrollable by a row select signal ROW SELECT for selectivelyconnecting the source follower transistor 58 and its output voltagesignal to a column line 62 of a pixel array.

FIG. 2 shows an exemplary CMOS imager circuit 200 that includes an array230 of pixels (such as the pixel 50 illustrated in FIG. 1) and a timingand control circuit 232. The timing and control circuit 232 providestiming and control signals for enabling the reading out of signals frompixels of the array 230 in a manner commonly known to those skilled inthe art. Exemplary arrays 230 have dimensions of M rows by N columns ofpixels, with the size of the array 230 depending on a particularapplication.

Signals from the imager 200 are typically read out a row at a time usinga column parallel readout architecture. The timing and control circuit232 selects a particular row of pixels in the array 230 by controllingthe operation of a row or vertical addressing circuit 234 and rowdrivers 240. Signals stored in the selected row of pixels are providedon the column lines 62 (FIG. 1) to a readout circuit 242 in the mannerdescribed above. The signal read from each of the columns is then readout sequentially using a horizontal/column addressing circuit 244.Differential pixel signals (Vrst, Vsig) corresponding to the read outreset signal and integrated pixel charge signal are provided asrespective outputs Vout1, Vout2 of the readout circuit 242.

FIG. 3 more closely shows the rows and columns 349 of a typical CMOSimager 300. Each column 349 includes multiple rows of pixels 350 (suchas the pixel 50 illustrated in FIG. 1). Pixel and reset signals from thepixels sensors 350 in a particular column can be read out to a readoutcircuit 352 associated with that column. The readout circuit 352includes sample and hold circuitry for acquiring the pixel and resetsignals. Signals stored in the readout circuits 352 are sequentiallyreadout column-by-column onto lines 370, 372 to an output stage 354,which in the illustrated example is common to the entire array of pixels330. The analog output signals Vout1, Vout2 may then be output, forexample, to a differential analog circuit that subtracts the reset andpixel signals and sends them to an analog-to-digital converter (ADC).Alternatively, the reset and pixel signals may each be supplied to theanalog-to-digital converter.

As can be seen from FIGS. 1-3, the CMOS image sensor chip normallyintegrates analog and digital components. As such, the analog circuitryinevitably suffers from substrate noise coupling. This is undesirablebecause substrate noise can compromise the signal-to-noise ratio of theimager. Substrate noise typically occurs when several circuits share thesame substrate and large transient currents from some of the circuitsare injected locally into the substrate through ohmic or capacitivecoupling.

Differential readout, i.e., taking the difference of separately readoutreset and pixel signals, is one way to reject common-mode substratenoise. In theory, any noise affecting the signals will be canceled whenthe signals are subtracted. Practically, however, since the reset andpixel signals are read out at different times, the noise may bedifferent at those times. As such, the imager may still suffer fromsubstrate noise.

One way of dealing with substrate noise is to use a dummy circuit,similar to a pixel circuit, located near the pixel circuit, but shieldedfrom light as a reference signal source. Theoretically, the pixel anddummy circuit would see the same substrate noise, which can then becorrelated by further processing. Unfortunately, the dummy circuit willcause a decreased fill factor (i.e., the ratio of light-sensitive pixelarea to total pixel area) for the pixels, and for some architectureswill cause an increase in KTC (thermal) noise.

Other proposed solutions include the use of complicated column and/orreadout circuitry, which are also undesirable. Accordingly, there is adesire and need for an imager pixel circuit that does not suffer fromsubstrate and other common-mode noise during a pixel readout operation,yet does not have increased complexity or fill factor.

SUMMARY

The present invention provides an imager pixel circuit that does notsuffer from substrate and other common-mode noise during a pixel readoutoperation and does not have increased complexity or fill factor.

The above and other features and advantages are achieved in variousembodiments of the invention by providing an imager in which twoadjacent pixels share row and reset lines and a row selection circuit.In addition, the output transistors of the two pixels are configured asa differential amplifier. In operation, both pixels are reset at thesame time, causing differential reset signals to be output from theamplifier. The charge from the first pixel is readout and a differentialpixel signal for the first pixel is output from the amplifier. Becausethe reset and pixel signals are differential signals generated withinthe pixels, they are free from common-mode noise. Correlated doublesampling can be used to obtain the pixel output value, which is alsofree from common-mode noise, from the differential reset and pixelsignals. The second pixel may be readout in the same manner. Because thetwo pixels are sharing circuitry, the pixels have decreased fill factorand complexity as well.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention willbecome more apparent from the detailed description of exemplaryembodiments provided below with reference to the accompanying drawingsin which:

FIG. 1 is a conventional image sensor pixel;

FIG. 2 is a block diagram of a conventional CMOS image sensor chip;

FIG. 3 is a block diagram of a conventional array of active pixels andassociated readout circuitry;

FIG. 4 is a portion of a CMOS imager constructed in accordance with anembodiment of the invention;

FIG. 5 is a timing diagram illustrating the operation of the FIG. 4imager; and

FIG. 6 is a portion of a CMOS imager constructed in accordance withanother embodiment of the invention;

FIG. 7 is a timing diagram illustrating the operation of the FIG. 6imager; and

FIG. 8 shows a processor system incorporating at least one CMOS imagerdevice constructed in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which are a part of the specification, and inwhich is shown by way of illustration various embodiments whereby theinvention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to make and use theinvention. It is to be understood that other embodiments may beutilized, and that structural, logical, and electrical changes, as wellas changes in the materials used, may be made without departing from thespirit and scope of the present invention.

Now referring to the figures, where like reference numbers designatelike elements, FIG. 4 shows a portion of a CMOS imager 400 constructedin accordance with an embodiment of the invention. The imager 400includes a two pixels 450 a, 450 b, a tail current source 410 and twooutput lines out1, out2. The two output lines out1, out2 are connectedto a respective load 414, 412. The two output lines out1, out2 are alsoconnected to the two pixels 450 a, 450 b. In the illustrated embodiment,the two pixels 450 a, 450 b are in the same column of an array, but arein adjacent rows. The two output lines out1, out2 serve as column outputlines for the two pixels 450 a, 450 b.

The first pixel 450 a includes a photosensor 452 a (e.g., photodiode,photogate, etc.), floating diffusion or storage node Na, transfertransistor 454 a, reset transistor 456 a and a source followertransistor 458 a. The photosensor 452 a is connected to the floatingdiffusion node Na by the transfer transistor 454 a when the transfertransistor 454 a is activated by a first control signal TX1. The resettransistor 456 a is connected between the floating diffusion node Na andan array pixel supply voltage. A shared reset control signal RESET isused to activate the reset transistor 456 a, which resets thephotosensor 452 a and floating diffusion node Na as is known in the art.The shared reset control signal RESET is also coupled to a resettransistor 456 b of the second pixel 450 b (the operation of which isdescribed below in more detail).

The source follower transistor 458 a of the first pixel 450 a has itsgate connected to the first pixel floating diffusion node Na. The sourcefollower transistor 458 a is connected between the second output lineout2 and node R connected to a shared row select transistor 460. Thesource follower transistor 458 a converts the stored charge at thefloating diffusion node Na of the first pixel into an electrical outputvoltage signal.

Similarly, the second pixel 450 b includes a photosensor 452 b (e.g.,photodiode, photogate, etc.), floating diffusion or storage node Nb,transfer transistor 454 b, reset transistor 456 b and a source followertransistor 458 b. The photosensor 452 b is connected to the floatingdiffusion node Nb by the transfer transistor 454 b when the transfertransistor 454 b is activated by a second control signal TX2. The resettransistor 456 b is connected between the floating diffusion node Nb andan array pixel supply voltage. The shared reset control signal RESET isused to activate the reset transistor 456 b, which resets thephotosensor 452 b and floating diffusion node Nb as is known in the art.

The source follower transistor 458 b of the second pixel 450 b has itsgate connected to the second pixel floating diffusion node Nb. Thesource follower transistor 458 b is connected between the first outputline out1 and node R connected to the shared row select transistor 460.The source follower transistor 458 b converts the stored charge at thefloating diffusion node Nb of the second pixel 450 b into an electricaloutput voltage signal.

The row select transistor 460 is connected between the tail currentsource 410 and node R. The current source 410 is preferably a MOSFEToperated as a current source, but may be any suitable source of current.The row select transistor 460 has its gate connected to a row selectsignal ROW SELECT. With the illustrated configuration, the sourcefollower transistors 458 a, 458 b form a differential amplifier 470 thatwill provide output signals over the output lines out1, out2 that arefree from substrate and other common-mode noise. For exemplary purposesonly, the differential amplifier 470 is configured to have a unity gain.It should be appreciated, however, that the amplifier 470 may beconfigured to have positive or negative gain if so desired. Theoperation of the differential amplifier 470 is described below in moredetail.

Furthermore, with the illustrated configuration, the two pixels 450 a,450 b and the shared row select transistor 460 and shared reset RESETand row select ROW SELECT lines form a differential pixel circuit 440.Due to the shared circuitry, the imager 400 has increased fill factorbecause less total area of the pixel 440 is being used in the non-lightsensitive portion of the pixel 440 (i.e., the ratio of light-sensitivepixel area to total pixel area is increased). The fill factor isimproved because the total pixel area is decreased or, if desired, theadditional area can be dedicated to the light-sensitive portion of thepixel 440. The differential pixel 440 has reduced complexity andcomponents as well.

The operation of the imager 400 is now described with reference to FIGS.4 and 5. When a row connected to the pixel 440 is to be read, a positiverow select signal ROW SELECT is applied to the shared row selecttransistor 460. This activates the shared row select transistor 460,which couples the current from the tail current source 410 to thedifferential amplifier 470 (via node R) within the pixel 440.

A reset operation is performed on both pixel 450 a, 450 b by pulsing thereset signal RESET. Once the reset signal RESET is pulsed, the two resettransistors 456 a, 456 b are activated at the same time. This causes thetwo pixels 450 a, 450 b to be reset at the same time. The differentialamplifier 470, having unity gain in the illustrated embodiment, causesthe first output line out1 to carry a differential signal having a valueequal to rst1−rst2, where rst1 is the reset signal of the first pixel450 a as it is applied to the first source follower transistor 458 a,and rst2 is the reset signal of the second pixel 450 b as it is appliedto the second source follower transistor 458 b. It should be appreciatedthat at this same time, the differential amplifier 470 causes the secondoutput line out2 to carry a differential signal having a value equal torst2−rst1 (described below in more detail).

Once the two pixels 450 a, 450 b are reset and the differential outputof the first output line out1 is sampled (by subsequent sample and holdcircuitry e.g., readout circuit 352 illustrated in FIG. 3), the firstcontrol signal TX1 is pulsed to activate the first transfer transistor454 a. The activation of the first transfer transistor 454 a causes thecharge stored in the photosensor 452 a of the first pixel 450 a to betransferred to the floating diffusion node Na and applied to the firstsource follower transistor 458 a. The differential amplifier 470, havingunity gain in the illustrated embodiment, causes the first output lineout1 to carry a differential signal having a value equal to sig1−rst2,where sig1 is the pixel signal of the first pixel 450 a as it is appliedto the first source follower transistor 458 a, and rst2 is the resetsignal of the second pixel 450 b as it is applied to the second sourcefollower transistor 458 b.

This second differential output from the first output line out1 issampled by sample and hold circuitry (e.g., readout circuit 352illustrated in FIG. 3). Correlated double sampling is then performed toobtain the difference between the two differential outputs, which isrst1−rst2−(sig1−rst2) or rst1−sig1. Thus, the first pixel 450 a isreadout using two differential signals, which are free from substrateand other common-mode noise. As such, the correlated double sampledresult (i.e., rst1−sig1) is also free from substrate and othercommon-mode noise typically found in conventional imagers.

After some row processing time, the second pixel 450 b is readout in asimilar manner. For example, the second pixel 450 b having already beenreset, can have the charge stored in its photosensor 452 b transferredto its floating diffusion node Nb by pulsing the second control signalTX2. The pulsing of the second control signal TX2 activates the secondtransfer transistor 454 a, which causes the charge stored in thephotosensor 452 b to be transferred to the floating diffusion node Nband applied to the second source follower transistor 458 b. Thedifferential amplifier 470, having unity gain in the illustratedembodiment, causes the second output line out2 to carry a differentialsignal having a value equal to sig2−rst1, where sig2 is the pixel signalof the second pixel 450 b as it is applied to the second source followertransistor 458 b, and rst1 is the reset signal of the first pixel 450 aas it is applied to the first source follower transistor 458 a.

This second differential output from the second output line out2 issampled by sample and hold circuitry (e.g., readout circuit 352illustrated in FIG. 3). Correlated double sampling is then performed toobtain the difference between the two differential outputs, which isrst2−rst1−(sig2−rst1) or rst2−sig2. Thus, the second pixel 450 b isreadout using two differential signals, which are free from substrateand other common-mode noise. As such, the correlated double sampledresult (i.e., rst2−sig) is also free from substrate and othercommon-mode noise typically found in conventional imagers.

The differential pixel 440 of the invention has been illustrated ascontaining two pixels 450 a, 450 b that share the row select transistor460 and utilize the same row select and reset lines. It should beappreciated that the differential pixel of the invention could includemore than two pixels. By using more than two pixels and the shared rowselect and reset circuitry, the differential pixel will have a fillfactor that is much greater than a fill factor of a typical imager pixeland that of the pixel 440 illustrated in FIG. 4.

It should be noted that the invention has been described with referenceto four-transistor (4T) pixels utilizing a transfer transistor. Itshould be understood that the invention may utilize a three transistor(3T) pixel, without the use of a transfer transistor. FIG. 6 is oneexample of a CMOS imager 600 constructed with 3T pixel circuits 650 a,650 b in accordance with an embodiment of the invention. The remainingcircuitry is substantially the same as the circuitry of imager 400illustrated in FIG. 4 except that the two reset transistors 456 a, 456 bare connected to receive respective reset signals RESET1, RESET2.

The operation of the imager 600 is now described with reference to FIGS.6 and 7. When a row connected to the pixel 640 is to be read, a positiverow select signal ROW SELECT is applied to the shared row selecttransistor 460. This activates the shared row select transistor 460,which couples the current from the tail current source 410 to thedifferential amplifier 670 (via node R) within the pixel 440. At thispoint, the differential amplifier 670, having unity gain in theillustrated embodiment, causes the first output line out1 to carry adifferential signal having a value equal to sig1−sig2. It should beappreciated that at this same time, the differential amplifier 670causes the second output line out2 to carry a differential signal havinga value equal to sig2−sig1. sig2−sig1 is sampled by the column sampleand hold circuitry.

A reset operation is performed on the first pixel 450 a by pulsing thefirst reset signal RESET1. Once the first reset signal RESET1 is pulsed,the first pixel 450 a is reset. The differential amplifier 670, havingunity gain in the illustrated embodiment, causes the first output lineout1 to carry a differential signal having a value equal to rst1−sig2.This second differential output from the first output line out1 issampled by sample and hold circuitry (e.g., readout circuit 352illustrated in FIG. 3). The same time sig2−rst1 is sampled by anothersample and hold circuit. Correlated double sampling is then performed toobtain the difference between the two differential outputs, which is(sig1−sig2)−(rst1−sig2) or sig1−rst1.

After some row processing time, the second pixel 450 b is readout in asimilar manner. sig2−rst1 is already stored in the column circuitry. Nowa reset operation is performed on the second pixel 450 b by pulsing thesecond reset signal RESET2. Once the second reset signal RESET2 ispulsed, the second pixel 450 b is reset. The differential amplifier 670,having unity gain in the illustrated embodiment, causes the secondoutput line out2 to carry a differential signal having a value equal torst2−rst1. This second differential output from the second output lineout2 is sampled by sample and hold circuitry (e.g., readout circuit 352illustrated in FIG. 3). Correlated double sampling is then performed toobtain the difference between the two differential outputs, which is(sig2−rst1)−(rst2−rst1) or sig2−rst2.

FIG. 8 shows system 800, a typical processor system modified to includean imager device 400 (FIG. 4), 600 (FIG. 6) of the invention. Examplesof processor systems, which may employ the imager device 400, 600,include, without limitation, computer systems, camera systems, scanners,machine vision systems, vehicle navigation systems, video telephones,surveillance systems, auto focus systems, star tracker systems, motiondetection systems, image stabilization systems, and others.

System 800 includes a central processing unit (CPU) 802 thatcommunicates with various devices over a bus 820. Some of the devicesconnected to the bus 820 provide communication into and out of thesystem 800, illustratively including an input/output (I/O) device 806and imager device 400, 600. Other devices connected to the bus 820provide memory, illustratively including a random access memory (RAM)804, hard drive 812, and one or more peripheral memory devices such as afloppy disk drive 814 and compact disk (CD) drive 816. The imager device400, 600 may be combined with a processor, such as a CPU, digital signalprocessor, or microprocessor, in a single integrated circuit.

The processes and devices described above illustrate preferred methodsand typical devices of many that could be used and produced. The abovedescription and drawings illustrate embodiments, which achieve theobjects, features, and advantages of the present invention. However, itis not intended that the present invention be strictly limited to theabove-described and illustrated embodiments. Any modification, thoughpresently unforeseeable, of the present invention that comes within thespirit and scope of the following claims should be considered part ofthe present invention.

1-59. (canceled)
 60. A method of operating a differential image pixelcircuit, said method comprising: simultaneously resetting twophotosensors; outputting a first differential signal corresponding to adifference between reset signals associated with the two resetphotosensors; transferring a first photo-generated charge from a firstof the two photosensors to a first storage node; and outputting a seconddifferential signal corresponding to a difference of a signal associatedwith the transferred first charge and a reset signal associated with asecond of the two photosensors.
 61. The method of claim 60, furthercomprising the step of using the first and second differential signalsto obtain a correlated double sampled signal.
 62. The method of claim 60further comprising the steps of: outputting a third differential signalcorresponding to a difference between reset signals associated with thetwo reset photosensors; transferring a second photo-generated chargefrom the second of the two photosensors to a second storage node; andoutputting a fourth differential signal corresponding to a difference ofa signal associated with the transferred second charge and a resetsignal associated with the first of the two photosensors.
 63. The methodof claim 62, further comprising the step of using the third and fourthdifferential signals to obtain a correlated double sampled signal.